Vibrational Energy Relaxation of Water Molecules in a Hydrated Lithium Nitrate Crystal

Water molecules in hydrated salts often have a well-defined geometrical arrangement and form an excellent model system for studying the effects of the hydrogen bond environment on vibrational energy relaxation. Hydrated lithium nitrate contains two distinct types of crystal water molecules. One water molecule makes strong and weak hydrogen bonds; the other water molecule makes two bifurcated hydrogen bonds. We use femtosecond two-dimensional infrared spectroscopy to probe the vibrational relaxation dynamics of the OD stretch vibration of dilute HDO molecules in lithium nitrate trihydrate. In the temperature range from 22 to 295 K we observe a decrease in vibrational lifetime from 3.8 +/- 0.2 to 2.8 +/- 0.1 ps for the strongly hydrogen-bonded species, from 5.41 +/- 0.08 to 4.14 +/- 0.05 ps for the bifurcated hydrogen-bonded species, and from 10.4 +/- 0.2 to 8.8 +/- 0.4 ps for weakly hydrogen-bonded species. This temperature dependence is opposite to that of the OD stretch vibration of dilute HDO:H2O ice, for which the 'vibrational lifetime increases from 480 +/- 40 fs at 25 K to 850 +/- 60 fs at 265 K. We discuss the origin of this difference in temperature dependence.